WZ Sagittae

WZ Sagittae

A visual band light curve for WZ Sagittae. The main plot, adapted from Kato (2015),[1] shows the period after the 2001 eruption, and the inset plot, adapted from Patterson et al. (2018),[2] shows the short-term variability including the eclipse.
Observation data
Epoch J2000      Equinox J2000
Constellation Sagitta
Right ascension 20h 07m 36.5036s[3]
Declination +17° 42′ 14.734″[3]
Apparent magnitude (V) 15.20[4]
7.0 - 15.5 B[5]
Characteristics
Spectral type DApe[6]
U−B color index +1.45[7]
B−V color index +1.49[7]
Variable type Dwarf nova[4]
Astrometry
Radial velocity (Rv)−51[4] km/s
Proper motion (μ) RA: 71.635[3] mas/yr
Dec.: -24.348[3] mas/yr
Parallax (π)22.1564 ± 0.0395 mas[3]
Distance147.2 ± 0.3 ly
(45.13 ± 0.08 pc)
Orbit
Period (P)82 mins[8]
Inclination (i)77 ± 2[9]°
Semi-amplitude (K1)
(primary)		40 ± 10[9] km/s
Details
A
Mass0.85±0.04[8] M☉
Surface gravity (log g)9.0[9] cgs
Temperature17,800[9] K
B
Mass0.078 - 0.13[8] M☉
Other designations
WZ Sge, Sge 1913, 2MASS J20073649+1742147, WD 2003+17.[4]
Database references
SIMBADdata

WZ Sagittae (WZ Sge) is a cataclysmic dwarf nova[4] star system in the constellation Sagitta. It consists of a white dwarf primary being orbited by a low mass companion. The white dwarf is about 0.85 solar masses while the companion is only 0.08 solar masses. This implies that the companion is a spectral class L2 star, although this has yet to be confirmed.[8] The distance to this system has been determined by parallax, yielding a distance of 45.1 parsecs.[10][3]

WZ Sagittae is an ultrashort period cataclysmic nova, with outbursts observed in 1913, 1946, 1978 and 2001. During the well-observed 2001 outburst, the nova reached a peak visual magnitude of 8.21. The 1913 event was the brightest of the observed outbursts, reaching a photographic magnitude of 7.0.[10]

This nova is classified as a SU Ursae Majoris class star, which is a subclass of dwarf nova that produces what are termed superoutbursts spaced several months apart, interspaced with normal outbursts every few weeks. The normal outbursts typically last 2−3 days, while a superoutburst lasts a few weeks. However, WZ Sagittae is unusual in that it is only observed to emit superbursts.[11]

The outbursts of a dwarf nova are caused when matter from a ring-like accretion disk becomes unstable. In this system, the companion star is sufficiently close to the white dwarf that the tidal bulge of the former overlaps the Roche limit, allowing matter to pass across and accumulate onto the disk. When the disk reaches a critical temperature, the gas collapses onto the white dwarf resulting in the release of gravitational potential energy.[12]

A superoutburst may be caused by a tidal interaction of the accretion disk with the donor star, resulting in a greater deposition of matter on the white dwarf. In the case of WZ Sagittae, however, magnetic effects may act to enhance the deposition, resulting in the lack of normal outbursts.[11] Because of the unique timing differences in the rate of superoutbursts of this nova, it been designated the prototype star for a WZ Sagittae subclass.

The orbital period of this system is 1.361 hours. Based upon observations of eclipses of the hot spot on the white dwarf (created by infalling material), the orbital plane of this system is inclined by 76° ± 6° to the line of sight from the Earth.[10]

  1. ^ Cite error: The named reference Kato was invoked but never defined (see the help page).
  2. ^ Cite error: The named reference Patterson was invoked but never defined (see the help page).
  3. ^ a b c d e f Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  4. ^ a b c d e "V* WZ Sge". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2009-09-03.
  5. ^ "GCVS Query=WZ Sge". General Catalogue of Variable Stars @ Sternberg Astronomical Institute, Moscow, Russia. Retrieved 2010-12-01.
  6. ^ Greenstein, J. L. (1975). "A further list of degenerate stars. VIII". Astrophysical Journal. 196: L117. Bibcode:1975ApJ...196L.117G. doi:10.1086/181758.
  7. ^ a b Krzeminski, W.; Kraft, Robert P (1964). "Binary Stars among Cataclysmic Variables. V. Photoelectric and Spectroscopic Observations of the Ultra-Short Binary Nova WZ Sagittae". Astrophysical Journal. 140: 921–935. Bibcode:1964ApJ...140..921K. doi:10.1086/147995.
  8. ^ a b c d Steeghs, Danny; Howell, Steve B.; Knigge, Christian; Gänsicke, Boris T.; Sion, Edward M.; Welsh, William F. (September 2007). "Dynamical Constraints on the Component Masses of the Cataclysmic Variable WZ Sagittae". The Astrophysical Journal. 667 (1): 442–447. arXiv:0706.0987. Bibcode:2007ApJ...667..442S. doi:10.1086/520702. S2CID 209833493.
  9. ^ a b c d Spruit, H. C.; Rutten, R. G. M. (1998). "The stream impact region in the disc of WZ SGE". Monthly Notices of the Royal Astronomical Society. 299 (3): 768–776. Bibcode:1998MNRAS.299..768S. doi:10.1046/j.1365-8711.1998.01809.x.
  10. ^ a b c Harrison, Thomas E.; Johnson, Joni J.; McArthur, B. E.; Benedict, G. F.; Szkody, Paula; Howell, Steve B.; Gelino, Dawn M. (2004). "An Astrometric Calibration of the MV-Porb Relationship for Cataclysmic Variables based on Hubble Space Telescope Fine Guidance Sensor Parallaxes". The Astronomical Journal. 127 (1): 460–468. Bibcode:2004AJ....127..460H. CiteSeerX 10.1.1.509.9281. doi:10.1086/380228. S2CID 45283092.
  11. ^ a b Matthews, O. M.; Speith, R.; Wynn, G. A.; West, R. G. (February 2007). "Magnetically moderated outbursts of WZ Sagittae". Monthly Notices of the Royal Astronomical Society. 375 (1): 105–114. arXiv:astro-ph/0611200. Bibcode:2007MNRAS.375..105M. doi:10.1111/j.1365-2966.2006.11252.x. S2CID 6180379.
  12. ^ "WZ Sagittae". Variable Star of the Month series. American Association of Variable Star Observers. September 2001. Retrieved 2009-02-21.